Pain circuitry in the spinal cord develops over a prolonged period of late fetal and early postnatal life and is particularly vulnerable to tissue trauma and inflammation during this time. Over the first few weeks of postnatal life, the central terminals of myelinated primary afferents undergo significant reorganization to generate the stereotypical laminar termination patterns seen in the adult dorsal horn. Recent findings in newborn mice have revealed that tactile afferents generate strikingly adult laminar termination patterns early on and thus undergo little central reorganization postnatally. By contrast, myelinated nociceptors give rise to widespread, inappropriate central projections in early postnatal life and thus the characteristic adult termination patterns of these afferents appear to mature later. Due to the relative immaturity of myelinated nociceptor central projections in newborns, the subsequent postnatal maturation of this group of afferents may be acutely susceptible to early perturbations of the periphery. The proposed experiments will examine the postnatal time course over which myelinated nociceptors achieve maturity to determine the potential window of this vulnerability. These studies will address the hypothesis that this early central exuberance is normally transient but that early tissue trauma, such as that caused by persistent tissue inflammation, arrests the normal postnatal reorganization of their central anatomy, leading to permanent structural alterations in spinal pain circuitry. An isolated somatosensory system preparation developed for comprehensive analyses of individual skin sensory neurons will be used to study the postnatal maturation of myelinated nociceptors in both hairy and glabrous skin of mice. A model of adjuvant-induced inflammation in glabrous skin of newborn mice will be used to examine the effects of early tissue trauma on the anatomical, physiological, and neurochemical maturation of individual neurons. Through intracellular recordings, neurons will be physiologically characterized using a variety of natural skin stimuli and then labeled in their entirety through iontophoresis of Neurobiotin. Their central projections will be reconstructed and analyzed morphometrically for changes taking place throughout postnatal maturation. Somata will be analyzed immunocytochemically to determine neurochemical changes, and peripheral endings will be analyzed for correlated structural changes in both normal and traumatized skin. The results of these studies will provide a detailed understanding of the early maturation of this vital component of the pain system, and the susceptibility of these afferents to early perturbation. This information will be pivotal to the development of future strategies in pediatric pain management and the translation of these strategies into effective practice.